1. Field of the Invention
This invention relates to a hydraulic brake system for a vehicle which performs anti-lock control, and more particular to a hydraulic brake system for a vehicle in which, with the aid of a control valve provided in a passage through which wheel brakes are hydraulically connected to a pressure generating chamber in a master cylinder, the wheel brakes are hydraulically isolated from the pressure generating chamber and hydraulically connected to a low back pressure reservoir, so that the brake fluid in the wheel brakes flows into the low back pressure reservoir, whereby the wheel brakes are decreased in hydraulic pressure; and with the aid of the control valve, the wheel brakes are hydraulically isolated from the pressure generating chamber and the low back pressure reservoir, and with the aid of a pump driven by an electric motor, the brake fluid in the low back pressure reservoir is caused to flow into the wheel brakes, so that the wheel brakes are reincreased in hydraulic pressure.
2. Description of the Related Art
A hydraulic brake system of this type is known in the art, being disclosed by Japanese Patent Unexamined Publication Nos. Sho 62-134361 and Hei 5-50910.
The hydraulic brake system disclosed by Japanese Patent Unexamined Publication No. Sho 62-134361 is designed as follows: With the aid of a control valve provided in a passage through which first and second wheel brakes are hydraulically connected to a pressure generating chamber in a master cylinder, the first and second wheel brakes are simultaneously hydraulically isolated from the pressure generating chamber and hydraulically connected to a low back pressure reservoir, so that the brake fluid in the first wheel brake and the brake fluid in the second wheel brake is caused to flow into the low back pressure reservoir, whereby the wheel brakes are decreased in hydraulic pressure at the same time. Furthermore, in the system, with the aid of the control valve, the first and second wheel brakes are hydraulically isolated from the pressure generating chamber and the low back pressure reservoir at the same time; and with the aid of a pump driven by an electric motor, the brake fluid in the low back pressure reservoir is caused to flow through a check valve into the first and second wheel brakes at the same time, so that the wheel brakes are reincreased in hydraulic pressure at the same time.
The hydraulic brake system disclosed by Japanese Patent Unexamined Publication No. Hei 5-50910 is designed as follows: Each of the first and second wheel brakes hydraulically connected through a passage to a pressure generating chamber of a master cylinder is provided with a low back pressure reservoir and a pump. A control valve comprising four two-port two-position type electromagnetic cut-off valves is able to individually hydraulically isolate the first and second wheel brakes from the pressure generating chamber and individually hydraulically connect them to the low back pressure reservoir, and it is able to hydraulically isolate the first and second wheel brakes individually from the pressure generating chamber and the low back pressure reservoir. The two pumps are driven by one electric motor, and the flow rate of brake fluid which is caused to flow into the wheel brakes by the pumps is smaller than the flow rate of brake fluid which is caused to flow from the wheel brakes into the low back pressure reservoir. With the anti-lock control hydraulic brake system thus designed, the first and second wheel brakes can be individually decreased or reincreased in hydraulic pressure.
On the other hand, there has been a demand for provision of an anti-lock control hydraulic brake system for a vehicle which is so designed that the hydraulic pressures of the first and second wheel brakes hydraulically connected to the pressure generating chamber of the master cylinder can be individually decreased or reincreased in hydraulic pressure, and which can be manufactured at low cost.
However, the above-described conventional hydraulic brake systems cannot meet this requirement. That is, the hydraulic brake system disclosed by Japanese Patent Unexamined Publication No. Sho 61-134361 is relatively low in manufacturing cost because it is relatively small in the numbers of electro-magnetic valves, pumps, and low back pressure reservoirs; however, it is unable to individually decrease or reincrease the hydraulic pressures of the first and second wheel brakes.
The hydraulic brake system disclosed by Japanese Patent Unexamined Publication No. Hei 5-50910 is able to individually decrease or reincrease the hydraulic pressures of the first and second wheel brakes; however, it is relatively high in manufacturing cost because it is large in the numbers of electromagnetic valves, pumps and low back pressure reservoirs.